Field of the Invention
The present invention relates to a lithographic apparatus.
Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
An object in a lithographic apparatus, for example a substrate table or a support structure for a patterning device, that moves in a gaseous environment can create an acoustic disturbance, so-called acoustic pressure waves, e.g. acoustic noise. An acoustic disturbance within the apparatus can cause disturbing forces that result in errors in the positioning of objects such as the substrate or the patterning device, which can in turn lead to overlay or other imaging errors. Such positioning errors can be caused by the acoustic disturbance acting directly on the object being positioned or indirectly, e.g. by the acoustic disturbance affecting measuring systems such as grid-encoder based or interferometer positioning systems or alignment sensors.
US 2012/0242271 A1 discloses an approach to minimizing the effect of noise on the positioning of an object table by sensing the noise and taking account of the sensed noise in control of the position of the object. It is also suggested that passive dampers such as Helmholtz resonators can be placed adjacent to the projection system to dampen vibrations at specific frequencies. However, these approaches do not address all acoustic disturbances that can occur.